Field of Invention
The present invention relates to a secondary battery electric power storage system and, more particularly, to a battery system which is prouided with evaluation meams of the soundness and the residual power of a battery, the distribution of the surplus electric power of a battery, and the optimum charge/discharge control of a battery. Most secondary batteries for automobiles, uninterrupted power supplies and the like have been lead-acid batteries.
The diffusion of electric power storage systems and electric vehicles, and the development of secondary batteries capable of storing electric power in a high energy density have been strongly desired to meet rising necessity for the leveling of power demand and progressively increasing demand for global environmental protection, and the development of new batteries has been expected.
Various kinds of large-capacity secondary batteries including nickelcadmium batteries, nickel-metal hydride batteries, lithium batteries and sodium-sulfur batteries will come onto the market in the future for various purposes. When managing batteries, appropriate charging and discharging of batteries according to their characteristics are essential to securing the soundness and the long life of batteries. Each battery has specific charge/discharge, temperature, rate and self-discharge characteristics. These characteristics vary from battery to battery.
In view of the convenience of using the battery, the residual capacity of the battery must be known as accurately as possible. Charging systems, temperature management and methods of determining the residual capacity for each of those batteries have been proposed. There have been known, for example, methods of detecting the residual capacity on the basis of only voltage (Japanese Patent Laidopen (Kokai) Nos. 58-85179 and 61-135335), a method of managing the residual capacity on the basis of voltage and current (Japanese Patent Laid-open (Kokai) No. 52-32542), methods of managing the residual capacity on the basis of current and time (Japanese Patent Laidopen Nos. 50-2130, 56-26271 and 59-28678), a method of managing the residual capacity on the basis of measured capacitance (Japanese Patent Laid-open (Kokai) No. 2-301974), a method of managing the residual capacity on the basis of voltage, current and temperature (Japanese Patent Laid-open (Kokai) No. 2-170372), a method of managing the residual capacity on the basis of internal resistance (Japanese Patent Laid-open (Kokai) No. 3-163375), a method of managing the residual capacity that integrates current and takes charge efficiency, discharge efficiency and temperature characteristics into consideration (Japanese Patent Laid-open (Kokai) No. 63-208773), and methods of determining the residual capacity on the basis of the specific gravity of the electrolytic solution (Japanese Patent Laid-open (Kokai) Nos. 56-24768, 57-88679 and 57-210578).
The capacity of a battery is dependent also on the charge and discharge history of the battery. For example, the capacity of nickel-cadmium batteries and nickel-metal hydride batteries decreases due to memory effect if shallow discharge and charge are repeated. In lithium batteries, lithium is accumulated on the positive electrode and material forms on the negative electrode to degrade the battery. If the charge/discharge balance is destroyed, the power of the lithium battery must be discharged after due consideration of the charge capacity. It is very important with combination batteries to know the charged capacity. Since the variation of the voltage of nickel-metal hydride batteries, as compared with that of nickel-cadmium batteries, in the final stage of charging is obscure, nickel-metal hydride batteries may be overcharged causing drying of the electrolyte or increase in the internal pressure and entailing safety problems unless nickel-hydrogen batteries are charged after due consideration of the discharge capacity. Since the capacity of batteries is greatly dependent on temperature, charging rate and discharging rate, the charge and discharge history of batteries must be taken into account. Charging systems have been studied with such problems in view. For example, a charging system disclosed in Japanese Patent Laid-open (Kokai) No.4-308429 charges a battery after detecting the fully discharged condition of the battery from the end voltage or time to solve problems due to memory effect, and a charging system disclosed in Japanese Patent Laid-open (Kokai) No. 61-81139 charges a battery after detecting the fully discharged condition of the battery from the end voltage or time to prevent overcharge.
A method of determining the residual capacity of a battery on the basis of the specific gravity of the electrolyte is applicable only to lead-acid batteries. Although a method of managing the residual voltage on the basis of the voltage is effective when applied to lead-acid batteries and lithium batteries the voltage of which varies comparatively widely during charging and discharging, the same is unsuitable for application to nickel-cadmium batteries and nickel-metal hydride batteries the residual capacity of which cannot be determined on the basis of only the voltage. It is difficult to predict the residual capacity accurately on the basis of current and temperature or on the basis of time besides on the basis of voltage under operating condition where discharge rate changes. When managing the residual capacity of a battery on the basis of measured internal resistance or capacitance, it is difficult to determine whether the increase of the internal resistance is due to the deterioration of the battery or whether the increase of the internal resistance is due to the exhaustion of the battery unless the mode of deterioration of the battery is precisely known. The management of the residual capacity on the basis of integrated current, charging efficiency, discharging efficiency and temperature characteristics allows one to predict the residual capacity considerably accurately, however, it is difficult to determine the residual capacity unless the capacity of the battery is known when the capacity of the battery is reduced greatly or the self-discharge of the battery is large. Since the prior art methods of determining the residual capacity of batteries are applicable only to specific batteries, respectively and hence each battery requires a specific control.
The system proposed to eliminate memory effect (Japanese Patent Laid-open (Kokai)No. 4-308429) and the system proposed to prevent overcharge (Japanese Patent Laid-open (Kokai) No. 61-81139) are intended to suppress the deterioration of a battery and to simplify a charging method when the battery is used as the power supply of portable apparatuses. Such a charge/discharge control method taking the characteristics and the history of batteries has not been proposed.
The difference between diurnal power demand and nocturnal power demand has progressively increased in recent years, and diurnal power demand in summer is approaching the upper limit of total power generating ability of power stations. Power storage techniques are effective means for solving problems attributable to the wide daily and seasonal variation of power demand. For example, electric power storage methods published in Denki Gakkai-shi, Vol. 111, No. 3, pp. 185-188 (1992) and such install large-capacity secondary batteries in a substation to store (to charge) surplus electric power generated in the nighttime and to deliver (discharge) the stored power when power demand reaches a peak in the daytime. However, nothing is mentioned about concrete means for the maintenance of soundness of the secondary batteries and the effective use of residual power.